In order to end up a Focus on 3:00 minute when doing your 3-min power your rest intervals must be at 0 W until HIE is filled up again. If your rest intervals are at LTP or so, your focus duration will increase. To rectifate this, you can use a higher power.
That being said, I do not think Magic Bucket uses the focus at all, but use the remaining Xhss, Xpss, and Xlss targets to give you interval tagets. So the power target will be based on the ratio between the remaining Xhss and Xpss. If you have run out of Xhss your target will be MPA. If you have run out of Xpss it will be just over TP.